Yesterday, Andrew Dessler graciously presented his viewpoint on the water vapor feedback (see). Today, I want to respond.

The first issue he raised is

1. Do observations indicate that the water vapor feedback strong and positive?

I completely agree with Andrew that, by itself, added water vapor is a positive feedback. We even see this on the local scale where minimum temperatures do not decrease at night as much when the air overhead is more humid.

However, the net effect of the water vapor feedback requires consideration of the other two phases of water (liquid and solid), in which our understanding is significantly incomplete. Andrew also raises this issue in his comment where he writes “[m]y opinion is that the cloud feedback is the only place where such a large negative feedback can lurk.”

The focus specifically on added CO2, though, as a source of a positive water vapor feedback ignores that any warming of the climate (such as from black carbon; e.g. see and see) must also necessarily result in such a positive water vapor feedback based on Andrew’s conclusions. Similarly, the human climate cooling forcings, such as from sulphates (e.g. see last paragraph), must result in a negative water vapor feedback. Even the diurnal variation of the Earth’s temperature (e.g. see) would result in positive and negative water vapor feedbacks within a year.

In terms of peer-reviewed papers which examine the water vapor feedback issue, Andrew too quickly dismisses the Wu et al 2009 paper. While this study does focus in the region 5°N-5°S, 150°E-110°W, this is in the El Nino/La Nina region where the relatively high sea surface temperatures means that the water vapor feedback is particularly amplified (evaporation, of course, is proportional to the exponent of temperature). Slight changes in temperature in this region have a disproportionately larger effect than the same temperature change would have when the water surface is cold.

There are also studies which do not show a concurrent warming and moistening of the atmosphere, at least on the regional scale; see

There is also a fundamental issue with overstating the role of water vapor as a positive feedback. If the feedbacks are positive, the resulting radiative imbalance should be greater than the sum of the radiative forcings. I discussed this, for example, in my posts

“One of the issues is whether climate feedbacks amplify or mute radiative forcings caused by human activities. The IPCC asserts that climate feedbacks in fact amplify the human effect. We can test this assertion using observational data.

If the magnitude of the IPCC estimates of radiative forcings from human causes are greater than or equal to the sum of the total observed radiative forcings and feedbacks (i.e. the total climate system radiative imbalance), then the feedbacks have actually reduced the effect of radiative forcings caused by human activities. By contrast, if the magnitude of radiative forcing caused by humans is less than the sum of the total observed radiative forcings and feedbacks than the feedbacks have amplified the human radiative forcings.

…….. the information that is used [to examine this] is

1. Total Radiative Forcing from Human Causes

The radiative forcings from human causes are provided by the 2007 IPCC Report [see page 4 of the Statement for Policymakers; Fig. SPM.2].

Their value is +1.6 [with a range of +0.6 to +2.4 Watts per meter squared]

This value, as reported in a footnote in the IPCC report, is supposed to be a difference with between current and pre-industrial values (but note that that this is not what is stated in the figure caption).

Thus, the total observed radiative forcing and feedback of 0.85 W/m^2 lies below the IPCC central estimate of 1.6 W/m^2 for just the human contribution to radiative forcing. This suggests that the climate feedbacks most likely act to diminish the effects of human contributions to radiative forcing, though it is important to recognize that a small part of the IPCC range (0.6 to 0.85) falls under the observed value from the work of Hansen.

This suggests that, at least up to the present, the effect of human climate forcings on global warming has been more muted than predicted by the global climate models.

This issue was inadequately discussed by the 2007 IPCC report. Climate Science has weblogged on this in the past (e.g. see), but so far this rather obvious issue has been ignored.

The second question is

2. Do models adequately reproduce the observed feedback?

There have been a number of studies which raise questions on the robustness of the IPCC-type models to skillfully represent the water vapor feedback. I reported on one study in my post

Model low, warm cloud optical and radiative properties are significantly different (biased) compared to those observed – two factors contribute to this extreme (bright) bias ‐ the LWP [liquid water path] is one, particle size is another.

Models contain grave biases in low cloud radiative properties that bring into question the fidelity of feedbacks in models

While I believe the changes that are likely to occur are primarily driven by changes in the large scale atmospheric flows, we have to conclude our models have little or no ability to make credible projections about the changing character of rain and cannot conclusively test this hypothesis.

The paper Wang et al 2009 that I posted on (see) on this subject is another study which raises serious issues with the modeling of the water vapor feedback.

Thus, the magnitude of the water vapor feedback, when clouds and precipitation are included, along with other climate system feedbacks, such as atmospheric-ocean interfacial fluxes, remains an incompletely understood subject.

I thank again Professor Dessler for engaging in a constructive dialog on this subject.